Spanx founder Sara Blakely at the launch of 'Haute Contour' by Spanx at Saks Fifth Avenue in 2009. Photo: Getty

You've probably got a pair in your underwear drawer, if not you can bet that at least one person within earshot of you does. Spanx, the control hosiery that promises to make you look like you've dropped a dress size, has become a household name across the globe thanks to their determined creator Sara Blakely, who at 41 years old, has just been named Forbes' youngest self-made female billionaire.

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Her company - founded with her $5,000 savings when she, like so many successful inventors, found the product she needed didn't exist - has just been valued at an average of $1 billion by four separate Wall Street investment banks making Blakely part of a tiny, elite club of American women worth ten figures.

Blakely's light bulb moment came when she bought a pair of cream trousers that she wanted to wear to a party, but hated the way her normal underwear gave her VPL. Wishing she could have the firming and smoothing effects of tights, she cut the feet off a pair and wore them underneath. Although this achieved the desired effect, the tights rolled up her legs all night, so she set her mind to creating a solution.

Her big breakthrough however came in 2000 when after sending samples to Oprah Winfrey's stylist she got a call from Harpo Productions warning her to get her website ready - Oprah had chosen Spanx for her annual 'Favourite Things Show'.

The rest, as they say, is history. Spanx now sells 200 products in 11,500 department stores, boutiques and online shops in 40 countries, with stockists clamouring to get added to the list. Their catalogue is mailed to over six million shoppers each year, and Sara Blakely's only just getting started.

Now, pardon us while we get back to our top secret plans for trousers that make your legs five inches longer...